Ductus arteriosus: A short vessel through which blood headed from the heart via the pulmonary artery to the lungs is shunted before birth. This blood is shunted away from the lungs and returned to the aorta. When the shunt is open, it is said to be patent. A patent ductus arteriosus (PDA) usually closes at or shortly after birth, and blood is permitted from that moment on to course freely to the lungs. If the ductus stays open, flow reverses, and blood from the aorta is shunted into the pulmonary artery and recirculated through the lungs. The PDA may close later on its own, or it may need to be ligated (tied off) surgically.